An offset printing press typically includes a plate cylinder carrying one or more printing plates. The printing plates have oleophilic surfaces defining an image area, and hydrophilic surfaces defining a non-image area. An inker applies ink to the printing plate which collects on the oleophilic surfaces to form an image which can be transferred to a blanket cylinder which transfers the image to media. Dampening solution may be applied to non-image areas. By transferring the image from the printing plate onto a blanket roller, and then onto the media, the printing plate does not directly print the image on the media, hence the term offset printing.
The inker applies ink carried on one or more form rollers to the printing plate. When the form roller in the inker engages the printing plate, the ink film on the form roller contacting image areas on the printing plate is split such that approximately one-half of the thickness of the ink film is applied to the image area of the printing plate leaving approximately one-half the ink on the form roller that never recovers its original ink film thickness on the printed, ink depleted areas causing a condition referred to as starvation. The ink film on the form roller contacting non-image areas on the printing plate remains on the form roller causing a condition called accumulation.
This combination of accumulation and starvation results in undesirable “ghosted” images and image repeats being formed on the final printed product. In order to minimize this problem, many conventional inkers include a plurality of form rollers (for example, four) which each apply a small amount.
The printed product is monitored to determine when ink density has degraded beyond an acceptable level. In order to control the quality of the printing, conventional printer inkers also include a plurality of adjustable keys to control the amount of ink being applied to the form roller. These keys require constant adjustment to maintain the quality of the printed product.
Keyless inking systems are generally known in the prior art. Some prior art keyless systems have attempted to solve “ghosting,” starvation, and accumulation problems in keyless inking systems employing single or multiple form rollers. However, these solutions have not been completely successful in solving these problems.
U.S. Patent Publication No. US2001/0032559, to Price et al, published on Oct. 25, 2001, now U.S. Pat. No. 6,672,211, discloses “Inking Systems for Printing Presses.” The content of this application is hereby incorporated by reference in its entirety. Embodiments disclosed in this patent publication include keyless inking systems with one relatively large form roller for applying ink to a printing plate. Ink is applied to the form roller by an applicator roller having an ink carrying surface and a variable speed drive. The form roller and plate cylinder are rotated at the same rpm while the speed of the applicator roller is varied to vary the amount of ink applied to the form roller. A subtractive roller system removes excess ink from the form roller. For wash-up, the press drive and form roller are disengaged and the inking system is rotated by an inker roller drive while wash-up fluid is applied to the inking system.
The systems of U.S. Patent Publication No. US2001/0032559 work well under some printing conditions using some conventional inks. Under other conditions, some improvement is possible. For example, some undesirable “tinting” in the non-image areas has been observed to occur when the system is used with some standard inks formulated for multiple form roll applications.
It is known in the printing arts to treat a printing plate so that hydrophilic, non-printing areas thereof are treated with a dampening fluid. Two general types of dampening systems are known: contacting and non-contacting. With the inker, a non-contacting spray brush or spray head system may be used in which the dampening fluid travels from a source or reservoir through a gap from the spray brush or spray head to a form or transfer roller within the inker. Such systems are typically used in lithographic systems for printing newspapers. In such systems an emulsion of ink and dampening fluid may be formed, and ink feedback into the dampening fluid reservoir may occur.
Dampening systems that are in physical contact with the inking system, especially in contact with the inker form roller, during the printing process are referred to as continuous duty dampeners. The dampening systems that incorporate a non-contact dampening fluid spray brush or spray head that sprays dampening fluid into the inker are referred to as non-continuous duty dampeners.
Keyless inking has been proposed for use in newspaper printing in combination with the use of spray brush or spray head dampening. Newspaper printers have generally avoided continuous dampening for a number of reasons, including the fact that the standard and keyless inking systems have no means of preventing uncontrollable accumulation of ink and fountain solution in the inkers resulting in lost color control and ink feedback into the dampening fluid reservoir.